The present invention relates to a method of Viterbi detecting and an apparatus for Viterbi detecting. More particularly, the invention relates to a method and apparatus which are suitable for Viterbi detecting by the use of a detection trellis that has a time-variant structure.
To transmit (or record) data, a block sync signal is usually added to the head of each data block (hereinafter referred to as lock). The block sync signal is detected when the data is received (or reproduced), thereby to detect the head of the block. The term lock, as used here, means a data unit that consists of a plurality of data words or code words. A block recorded on DAT (Digital Audio Tape), for example, is composed of a 1-symbol ID (Identity) data item, a 1-symbol ID parity and a 32-symbol data word. That is, this data block consists of 36 symbols. Each symbol consists of 8 bits before it is encoded to an 8/10 recording code, and by 10 bits after it is encoded to an 8/10 recording code. To record the block, the 8-bit symbols forming the block are encoded to 8/10 codes. Thereafter, a sync signal (sync word), which is a 1-symbol (10 bits) and which does not exist in the sequence of 8/10 codes, is added to the head of the block. To reproduce the block, the sync word is detected, thus finding the head of the block. Then, the ID data, parity and 32-symbol data word are decoded, symbol by symbol. The 8/10 codes have been generated to accomplish NRZI (nonreturn-to-zero interval) recording. The maximum length Tmax of a train of identical bits, existing in the NRZI-converted sequence of 8/10 codes, is 4. Two or more identical bit trains having Tmax would not follow one after another. The sync word contains a pattern in which Tmax continuously appears at least twice, though Tmax does not appear in the sequence of codes. The Tmax distinguishes the sync word from the sequence of codes.
In recent years, the TCPR (Trellis-Coded Partial Response) system has been studied with enthusiasm in the field of magnetic recording. In this system, the partial response characteristic and the code characteristic are integrated, thereby to increase the Euclidean distance between the output sequences on a transmission path (recording/reproducing path), i.e., free square Euclidean distance d2free. An increase of distance d2free is equivalent to a rise of signal level. Hence, the TCRP system enhances SNR (Signal-to-Noise Ratio) at the time of detecting data. Codes used in the TCRP system are generally called Trellis signals. The assignee of the present application has proposed 16/20 trellis codes in Japanese Patent Application No. 10-207372.
FIG. 5 shows an ADS trellis illustrating the transition of an ADS (Alternating Digital Sum) that a 20-bit code word assumes. The ADS can range from 0 to 10 for the code word. However, the ADS at the start point and the ADS at the end point can take only two values, i.e., 3 and 7. Further, the ADSs at time 7 are limited to the values shown in FIG. 5, in accordance with whether the value of the ADSs at the start points is 3 or 7. An ADS is the sum of symbols acquired to the present time, from a time in the infinite past or from the start of encoding the symbols, each symbol being xe2x88x921 or +1 allocated to one code bit, and every other symbol having been multiplied by 1. If the ADs for a sequence of codes is set within a specific range, the Nyquist frequency component of the code power density, i.e., the frequency component which is half (xc2xd) the code bit rate, can be reduced to null.
It is known that the distance d2free increases when the null point of the code power density is made to coincide with the null point of the transfer function of the transmission path. The power density of a 16/20 code has the null point at the Nyquist frequency. This code is therefore a trellis signal to a partial response whose transfer function has a null point at the Nyquist frequency.
FIG. 6 depicts a detector trellis that is used to Viterbi detect in the TCPR system, by the use of a class-1 partial response. The detector trellis has a structure, which is a combination of the characteristic of the code and the characteristic of the class-1 partial response. (The characteristic of the code is the ADS trellis of FIG. 5, hereinafter referred to as code trellis. The transfer function of the class-1 partial response has a null point at the Nyquist frequency.) The structure shown in FIG. 6 is time-variant in units of code words. This is why a sequence can be detected in the TCPR system only when the boundary between the adjacent code words is determined.
FIG. 7 shows a reproduction apparatus of a TCPR system, in which a time-variant trellis is utilized. The signal reproduced from a medium 31 and amplified by a regenerative amplifier 32 is equalized to a prescribed partial-response characteristic by means of an equalizer 33. Then, a PLL 34 extracts a clock signal from the output signal of the equalizer 33. The PRML(Partial Response Maximum Likelihood) Viterbi detector 36, sync word detector 37, TCPR Viterbi detector 38 and decoder 39, all incorporated in the reproduction apparatus, operate in accordance with the clock signal. The signal equalized to the prescribed partial-response characteristic and output from the equalizer 33 is sampled by a sampling circuit 35 and then input to the TCPR Viterbi detector 38 via a delay element 310. The PRML Viterbi detector 36 detects, using a detector trellis that has no characteristics of a time-variant trellis. Therefore, the PRML Viterbi detector 36 need not determine the boundary between adjacent code words and can detect data asynchronously. The bit train the PRML Viterbi detector 36 has detected is input to the sync word detector 37. The sync word detector 37 detects a sync word from the bit train and supplies the sync word to the TCPR Viterbi detector 38. The TCPR Viterbi detector 38 detects, using a detector trellis that has the characteristics of a time-variant trellis (i.e., the trellis shown in FIG. 6). Hence, the TCPR Viterbi detector 38 cannot correctly operate unless the boundary between adjacent code words is determined. The TCPR Viterbi detector 38 is thereby synchronized by the input sync word and starts detecting one block of data. The data thus detected by the TCPR Viterbi detector 38 is input to the decoder 39, which decodes code words. The delay element 310 is provided to delay the output of the equalizer 33 by the same time the sync word detected has delayed due to the internal delays of the PRML Viterbi detector 36 and sync word detector 37.
The accuracy of detecting the block sync word greatly influences the reception of data or the quality of the data reproduced. For example, one block of data will be lost in its entirety if the sync signal is not detected due to disturbance. If the sync signal is detected incorrectly, data will be lost for the period between the time when the sync signal is incorrectly detected and the time when the sync signal is correctly detected. In the case of DAT (Dynamic Address Translation), the ID data will be erroneously detected, too, if the sync word is detected incorrectly. The ID data contains address data. If the incorrectly detected address data is used, thereby writing data into a memory, the data of any other block may be destroyed.
The characteristics of a trellis code are not imparted to the PRML Viterbi detector 36 shown in FIG. 7. Therefore, the reproduction apparatus of a TCPR system cannot obtain a gain. It follows that the output from the PRML Viterbi detector 36 is inevitably inferior to the output of the TCPR Viterbi detector 38 in terms of quality.
No TCPR system is employed in DAT. Only one data detector is used, and the accuracy of detecting a sync word is the same as that of detecting data. In the reproduction apparatus of FIG. 7, however, the accuracy of detecting the sync word is much lower than that of detecting data. This will impose a very adverse influence on the reception of all data or the quality of the data reproduced, as mentioned above. It is therefore desired that the accuracy of detecting the sync word be rendered sufficiently high by any means.
In the reproduction apparatus of FIG. 7, the sync word is used for synchronizing the TCPR Viterbi detector 38. Thus, the sync word need not have such a time-variant trellis structure as is illustrated in FIG. 6. There will be some advantage, however, if the sync word has the time-variant structure. One of the advantages lies in that the sync word can be detected from the output sequence of the TCPR Viterbi detector 38, too, without changing the operation mode of the detector 38, provided that the sync word has the time-variant trellis structure that the TCPR Viterbi detector 38 uses. The result of detecting this sync word is so reliable that it can be applied to verify the sync-word detection signal obtained from the output sequence of the PRML Viterbi detector 36. If the sync word does not maintain the detection trellis structure that the TCPR Viterbi detector uses, the sync work cannot be detected by means of TCPR Viterbi detection. Therefore, it is impossible to detect sync signals from the output sequence of the TCPR Viterbi detector 38.
Another advantage lies in that TCPR Viterbi detection can be started at the head of the sync word if the sync word has a time-variant structure. This helps the likelihood to converge in the Viterbi detection. Viterbi detection is one of maximum likelihood decoding methods. In the Viterbi detection, the likelihood of every state in the detector trellis is examined, thereby determining the most likely sequence. Assume that the Viterbi detection is started with the data immediately following the sync word at the very time the TCPR Viterbi detector 38 is synchronized. Right after the TCPR Viterbi detector 38 is synchronized, the likelihood of each state in the detector trellis does not always have an appropriate value. This is because the likelihood of any state, which is obtained while the detector 38 remains asynchronous, is insignificant. In the case where the likelihood does not have an appropriate value, the detection accuracy will decrease until the likelihood converges to a desirable value, if the Viterbi detection has been started with the data. Nonetheless, if the Viterbi detection is started with the sync word, it is possible to save time for the convergence of likelihood, and it can be expected that the likelihood has converged at the start of detecting the data.
The likelihood of each state at time 0 in the detector trellis may be initialized at the very time of synchronizing the TCPR Viterbi detector 38. To this end, it is necessary to determine in which state at time 0 the Viterbi detection should be started. The likelihood of the state can be set at a high value if the state of the start point of the detector trellis is known. Thus, data can be detected with high reliability from the beginning, well before the likelihood converges.
The Viterbi detector may be already synchronized with the output sequences on the transmission path, which has been input to the Viterbi detector. In other words, the Viterbi detector and the output sequence on the transmission path may be synchronous with each other. If this is the case, it is not always necessary to set the likelihood every time a sync signal is detected. Rather, the likelihood may possibly be distorted.
Accordingly, the object of this invention is to provide a method of Viterbi detecting and an apparatus for Viterbi detecting, both capable of Viterbi detecting without distorting the path-metric value by applying a prescribed fixed metric.
According to the present invention there is provided a method of Viterbi detecting for detecting a code sequence containing a sync word, from an output sequence on a transmission path, by using a detection trellis that has a time-variant structure. The method is characterized in that the likelihood of a state in which the sync word starts or ends in the detection trellis is initialized only if the time the sync word is detected in the detection trellis does not coincides with a time extrapolated in the detection trellis before the sync word is detected.
According to the invention there is provided an apparatus for Viterbi detecting, designed to detect a code sequence containing a sync word, from an output sequence on a transmission path, by using a detection trellis that has a time-variant structure. The apparatus comprises: a base counter for measuring time in the detection trellis; a comparator for comparing the time when the sync word is detected, with the time indicated by the base counter, and for outputting a coincidence/non-coincidence signal; and a selector for selecting and outputting an output of an ordinary ACS (Add Compare Select) circuit and a predetermined initial metric value. The selector selects the output of the ordinary ACS circuit when the output of the comparator indicates coincidence, and selects a prescribed initial metric value when the output of the comparator indicates non-coincidence.
The present invention uses a detection trellis having a time-variant structure. To detect a code sequence containing a sync word, from an output sequence on a transmission path, the likelihood of a state in which the sync word starts or ends in the detection trellis is initialized only if the time the sync word is detected in the detection trellis does not coincides with a time extrapolated in the detection trellis before the sync word is detected. Viterbi detection can thereby be accomplished, without changing the value of the path-metric by applying a prescribed fixed metric.
Hence, the present invention can provide a method of Viterbi detecting and an apparatus for Viterbi detecting, which can Viterbi detect without distorting the path-metric value by applying a prescribed fixed metric.